The bus driver died in the wreck, making it difficult for police to immediately determine the cause, said R.M. Sharma, the superintendent of police in Shimla -- the district in the Himachal Pradesh state where the incident occurred.

Shimla is about 370 km (229 miles) from the capital city, New Delhi.

Deep Ram, head police constable in Shimla, said roads in the area are narrow and winding and the driver may have lost control.

Deadly road crashes are common in India and often result from overcrowded and old buses coupled with speeding drivers.

In October, a bus carrying 67 workers crashed into a parked truck in the northeast state of Assam, killing at least 23 people.

And in September, 14 people died when a bus crashed into a roadside tent while trying to round a curve in the eastern state of Bihar.